Quantum dots (QDs) are a unique class of semiconductors because they are small, ranging from 1-10 nm (10-50 atoms) in diameter. At these small sizes, materials behave differently from bulk semiconducting materials, giving quantum dots unprecedented tenability and enabling never before seen applications to science and technology. As compared with bulk semi-conducting materials, quantum dots show a more tunable range of emission energies. With their high luminescence quantum efficiency and acceptable lifetime, QDs can be classified as a promising material for display application.
The concepts of energy levels, band gap, conduction band and valence band still apply in quantum dots. However, they are different from bulk materials.
i. Quantum Dots' Electron Energy Levels are Discrete Rather than Continuous.                The addition or subtraction of just a few atoms to the quantum dot has the effect of altering the boundaries of the band gap. Changing the geometry of the surface of the quantum dot also changes the band gap energy, owing again to the small size of the dot, and the effects of quantum confinement.        
ii. Quantum Dot Materials Show Tunable Band Gap.                The fixed energy gap of bulk semiconductor and the controllable energy gap of quantum dots are displayed in FIGS. 1a-1c. It can be seen that with the decrease of quantum dot size from QD1 to QD3, the band gap can be tuned from Eg(QD1) to Eg(QD3), and the color of the quantum dots can be relatively tuned from red to blue (FIG. 1b). As with bulk semiconductor materials, electrons tend to make transitions near the edges of the band gap (FIG. 1c). It is therefore possible to control the output wavelength of a dot with extreme precision. In effect, it is possible for us to tune the band gap of a dot, and therefore specify its “color” output depending on our needs.        
iii. Quantum Dot Display with Good Color Saturation                Quantum dots, suspended in liquid, absorb certain light and then reemit it in a specific color that depends on the particle's size. Each quantum dot is about one ten-millionth of an inch in diameter and is composed of a few hundred atoms of material and the colors of light they produce are much more saturated and controllable than that of other source, which is very meaningful for lighting.        
iv. Quantum Dot Materials Show Free of Light Scattering.                With quantum dot technique, a tailored-spectrum white light is available. What is more, the small size of the quantum dots, which is much smaller than the wavelength of visible light, can eliminates all light scattering and the associated optical losses. In contrast, optical backscattering losses using larger conventional phosphors reduce the package efficiency by as much as 50 percent.        
In the recent decades, QD-LED products are available in the market. However, one of the most serious obstacles for using QD-LEDs in consumer products is the toxicity of Cd- and Pb-based QDs. Core-shell type QDs have been developed to address this issue. However, the sealing technology and how to lower down the production cost remain big challenges for the existing QD-LEDs.
Furthermore, commercially available white LEDs emit a harsh and bluish cold white light, with poor color rendering properties, which limit their wide-scale use in indoor illumination applications. The poor quality of the white light perceived originates in the yellow converter material, Ce:YAG, due to its lack of emission in the green and red parts of the spectrum as shown in FIGS. 2a and 2b. The broadband light emitted by the Ce:YAG phosphor are roughly 500-700 nm. Correction of emission of conventional phosphors is needed.
In short, there remains a need to develop a cost effective and green approach for large scale synthesis of non-toxic luminescent quantum dots with precisely controlled luminophore and to improve emission of phosphors.